1. Field of the Invention
The present invention relates to a spectrometer objective for electron beam mensuration techniques for quantative measurements of potential in which an objective lens for focusing a primary electron beam onto a specimen and an electrostatic opposing field spectrometer are provided, the arrangement comprising an electrode arrangement for the extraction of the secondary electrons triggered at the specimen by the primary electron beam and an electrode arrangement for generating an electrical opposing field which retards the secondary electrons from an electron optical unit. A detector arrangement is also provided for documenting the secondary electrons.
2. Description of the Prior Art
Conventional scanning electron microscopes equipped with beam blanking systems and opposing field spectrometers are currently utilized for quantitative measurements of potential at nodes and interconnects in large scale integrated (LSI) circuits. However, adequately fine electron probes for the examination of very large scale integrated (VLSI) circuits having structures in the sub-micrometer range cannot be produced with modified scanning electron microscopes, since these devices must be operated at low primary electron energies in order to avoid radiation damage to and charges at the components that are usually arranged on insulating carrier substances. A noticeable improvement of the spatial resolution that is essentially limited by the axial chromatic aberration of the objective lens and by the electron-electron interaction (Boersch effect) can only be achieved by way of a short electron optical beam path having few beam crossover points and an objective lens having a short focal length. Up to now, the use of short focal length objective lenses for the reduction of the axial chromatic aberration increasing proportionally with the focal length ran aground of the structure of electron beam measuring installations wherein a secondary electron spectrometer is arranged between the objective lens and the specimen.
Only with the development of objective lenses having an integrated secondary electron spectrometer (spectrometer objective) could the aberration of the objective lens be reduced with the working distance and, therefore, the probe diameter diminshed in size on the specimen. Such a spectrometer lens is known from the publication of Kawamoto entitled "Electron Beam Tester with In-the Lens-Analyzer", in the Proceedings of the Symposium on Electron-Beam-Testing, November 9-10, 1984, Osaka, Japan, pp. 69-72.
This known arrangement is a matter of a magnetic objective lens having a short focal length and comprising an integrated parallel plate analyzer and an electrode arranged above the objective lens for the deflection of the secondary electrons in the direction of a detector.
Although the diameter of the electron probe on the specimen can be noticeably reduced in comparision to conventional arrangements when an electron beam measuring apparatus equipped with the known spectrometer lens is used, the spatial resolution of these devices remains limited. The reason for this is the Coulomb repulsion of the electrons in the beam path between the electron source and the specimen, this Coulomb repulsion opposing a focusing of the electron beam.